Fungicide resistance and osmotic stress sensitivity in os mutants of Neurospora crassa

The os-1 (osmotic sensitive-1) mutants in Neurospora crassa were classified into two groups based on their resistance to various Fungicides (e.g., flu dioxonil, iprodione, and quintozene) and osmotic stress. The first group of strains, represented by NM233(t) and Y256M209, grew on medium containing f ludioxonii even at a concentration of 7-5 mu g/ml but their growth was inhi bited only when a high concentration of NaCl was used for osmotic stress. T he second group of strains, represented by M155-1, M16, P3182, and P59901 w as less resistant to fludioxonil than the first group, but their growth was inhibited even under low osmotic stress. Both groups of the os-1 mutant st rains transformed by the wild-type os-1 gene restored fungicide sensitivity and osmotic tolerance, suggesting that the os-1 gene mutations are respons ible for phenotypes in both groups of os-1 mutants. The results indicated t hat the level of resistance to fludioxonil is not related to that of osmoti c sensitivity in the os-1 mutant strains. To investigate the correlation be tween fungicide resistance and osmotic sensitivity in other os mutant strai ns, we isolated revertant strains from the os-5 mutant strain on medium con taining 6% NaCl. The revertant os-5R-18 strain reversed its osmotic sensiti vity to the level of the wild-type strain but still remained resistant to f ludioxonil. Genetic analysis suggested that an additional mutation, named s u(os-5), in the revertant os-5R-18 strain suppressed the, osmotic sensitivi ty of the os-5 mutant strain but did not do so with those of the os-1 and o s-2 mutant strains. The os-5 mutant strain accumulated less glycerol by osm otic stress, while glycerol synthesis in the revertant os-5/su(os5) strain was induced by high osmolarity to the level of the wild-type strain. Howeve r, glycerol accumulation by fludioxonil observed in the wild-type strain di d not occur in either strain with os-5 and os-5/su(os5). These results sugg ested that phenylpyrroles affect glycerol synthesis in the osmotic signal t ransduction pathway, bur that high osmotic sensitivity is not directly link ed with resistance to dicarboximides and phenylpyrroles. (C) 2000 Academic Press.